The production of powdered metal articles, including gears, is well-known in the art. One can use a single type of powdered metal or, depending upon the properties desired, one can use different types metal powders blended together. The powder is disposed in a mold cavity which can be a simple cylindrical preform or may have the profile of the finished product. Then, pressure is applied to create the preform. The preform can then be removed and sintered to produce the part. Where a cylindrical preform is used, the preform is placed in another mold and more pressure is applied to form an article having the desired shape. This new preform can then be sintered.
Apparatus for forming helical gears are also known in the art wherein portions of the mold rotate when the preform is impacted to cause the preform to take the shape of the helical gear. For example, such an apparatus having rotating parts for producing powdered metal helical gears is disclosed in U.S. Pat. No. 3,891,367 to Signora. In Signora, the preform has the shape of the actual helical gear to be produced, in contrast to first forming a cylindrical preform which is later transformed into a helical gear.
U.S. Pat. No. 4,712,411 to Goodwin discloses an apparatus for making a fully dense powdered metal single helical gear. Goodwin generally describes producing the helical gear by first creating a cylindrical preform by sintering. The cylindrical preform is then placed in a forming mold wherein the mold cavity has the specific geometry of the helical gear. The preform is then heated and placed in the forming mold where it is axially impacted with an unprofiled punch to both impact the helical toothed shape and also to densify the gear. A disadvantage of the method employed by Goodwin can be that when the preform is impacted a lot of flashing can result as the preform is forced into the shape of the helical gear and the metal passes through the gap of the unprofiled punch and the profile of the teeth. Consequently, additional finishing processes can be required to clean up the gear before it is acceptable to a customer.
Both Signora and Goodwin utilize mechanically created pressure to form the gear. However, it is also known to utilize isostatic pressure to form a helical powdered metal gear. For example, U.S. Pat. No. 5,390,414 to Lisowsky discloses a method of manufacturing a helical gear from powered metal using hot and cold isostatic pressure. Like Goodwin, Lisowsky employs a first mold to create a simple cylindrical preform having only the general geometry of the intended gear. A second mold is provided having the specific geometry of the gear and is slightly larger than the preform. The preform is placed inside the second mold, wherein additional powdered metal is provided around the preform to produce a second preform having a helical gear shape. Cold isostatic pressure is used to create both the simple preform and the helical gear preform. After the helical gear preform is made, hot isostatic pressure and/or sintering is employed to create the densified helical gear.
Isostatic pressure forming can generally involve placing a gear preform within a mold cavity having the specific geometry of the helical gear. A rubber bladder is inserted through a center bore in the gear. Fluid is pumped into the rubber bladder at extremely high pressures thus radially expanding the preform against the walls of the mold cavity and causing it to take on the helical gear shape. A disadvantage with isostatic forming is that it can take much longer for the process to fully densify the gear. In hot forming, enormous amounts of pressure can be generated in an instant by impacting the gear axially. In contrast, with isostatic pressure it can take time to build up the pressure and it may be preferable to keep the gear subjected to the pressure for a relatively long time to ensure that the preform fully takes on the specific geometry of the helical gear. Also, for example, obtaining accurate dimensions in the axial direction can be difficult when using isostatic pressure forming. There is generally no mold abutting the axial ends of the gear because the bladder must be inserted through a center bore in the gear. Thus, the axial dimension can be difficult to accurately control. Consequently, more finishing steps can be required to obtain final dimensions having the desired accuracy. Moreover, besides controlling the length of the gear, the lack of control over the axial dimension can also make it more difficult to fully densify the gear. This is because without control over the axial dimension, the gear can experience some undesirable axial expansion in addition to the radial expansion. Consequently, instead of compacting all of the molecules of the gear together, as would occur if both the radial and axial dimensions were controlled, the gear lengthens somewhat which results in a longer and less dense gear.
U.S. Pat. No. 6,592,809 to Anderson discloses a method for producing a fully dense powder metal single helical gear including placing powder metal in a preform die wherein it can be compacted axially by punches to create a gear preform having various gear profiles, such as a helical profile, sintering the preform, and inserting the sintered preform into a hot forming die wherein it is impacted axially by punches to fully densify the gear preform. The densified gear can then be inserted in a burnishing die where a more precise gear profile can be imparted resulting in more precise dimensions. Anderson further indicates that finishing treatments, such as rolling, shaving, heat treating, machining to length and inner diameter sizing can be subsequently performed as desired.
In comparing common types of power transmission gears, a helical gear which has angled teeth typically has a higher load carrying capacity than the common spur gear, which has straight-cut teeth, of the same size. Also, because the helical gear runs more smoothly than the correspondingly spur gear, helical gears can normally operate at much higher speeds than can spur gears. However, in operation helical gears also create an axial thrust which may have to be absorbed by the bearings if same are employed to carry the gear. Relatively expensive bearings such as thrust bearings are normally employed to absorb such axial forces.
The advantages of a helical gear can be obtained, without the requirement of thrust bearings, however, through use of a dual helical gear known as a herringbone gear. In general, a dual helical gear has a helical profile on one side of a parting plane and an opposite helical profile on the other side of the parting plane. These two profiles can be an opposed helix, a helix of different pitch of the same hand, or out of phase helix, or a spur gear, whether of the same diameter or tooth height or not. The special case of symmetrically opposed helical gears is referred to as a herringbone design.
A herringbone gear is constructed of two adjacent rows of helical or angled gear teeth which extend around the circumference of the gear with the two rows of teeth being oppositely angled. By providing the two rows of oppositely angled gear teeth, axial forces normally created by a helical gear, are avoided. Any axial thrust created by one row of angled teeth is cancelled by the opposite acting reaction axial thrust created by the other row of angled teeth. Overall, herringbone gears possess advantages over other gear types in that (1) there is a continuous smooth meshing of gear teeth; (2) they afford greater strength; (3) there is the absence of end or axial thrust as noted above; (4) they may be operated at high peripheral speeds; and (5) they possess the ability to withstand shock loads and loads of a vibrating nature because of very low backlash.
Herringbone gears, however, requiring complex machine tools and are difficult and costly to manufacture. Consequently, the cost of herringbone gears is quite high. In fact, though herringbone gears are often preferred in certain mechanical applications, cost has historically prevented their widespread utilization and lesser desirable gears have been employed in their place.
Since it is difficult and costly to manufacture dual helical gears such as herringbone gears utilizing conventional technology, the advantages of manufacturing such gears by compacting and sintering a powder metal are readily apparent. U.S. Pat. No. 6,165,400 to Hinzmann discloses such as technique. More specifically, Hinzmann reveals a method of forming a powder compact in a tool set having a pair of opposed die and punch sets each having a die and a punch co-operating with said die to define respective chambers, said method comprising the steps of: (a) establishing said tool set in a closed position, with said chambers in closed communication to form a closed cavity containing a charge of powder; (b) advancing said punches toward each other along an axis to compress the charge of powder and thereby to form the compact; (c) maintaining said punches at a fixed spacing from each other while moving each of said dies along the axis to separate the dies and expose the compact; (d) rotating at least one of said dies about the axis as it moves along said axis; and (e) ejecting the compact from said tool set. However, the gears made by the powder metal technique of Hinzmann are not of high density.